Method for data communication via a voice channel of a wireless communication network using continuous signal modulation

ABSTRACT

A system and method for data communication over a cellular communications network that allows the transmission of digital data over a voice channel using a vocoder that monitors parameters of a Levinson Durbin recursion and then uses full rate CELP if the monitored prediction error falls to below a predetermined threshold within a pre-selected number of iterations of the recursion. The system and method encode digital data to be transmitted using a continuous signal modulation technique at a selected bit rate and one or more frequencies that are selected such that the resulting modulated carrier signal is processed by the vocoder using full rate CELP as a result of the monitored prediction error.

TECHNICAL FIELD

The present invention relates generally to data communication over atelecommunications network and, more particularly, to data communicationover a telecommunications voice channel using an EVRC vocoder.

BACKGROUND OF THE INVENTION

Wired telephone systems were originally designed to carry speech toenable voice conversations over long distances. More recently, publicswitched telephone systems have become a primary medium for transmittingnot only voice, but also non-speech data, such as by use of facsimilemachines that transmit image information over the telephone lines, or bymodems that exchange digital data of various forms (text, binaryexecutable files, image or video files) over these same phone lines.

Today, cellular and other wireless communication systems are in muchgreater use for purposes of both voice and data communication. Mostcellular communication in use in the world today utilize either the GSM(including UMTS) or CDMA (IS-95 or CDMA2000) communication systems.These systems transmit voice data over a voice traffic channel using amodulated carrier wave. For example, 2G GSM uses GMSK modulation andIS-95 CDMA uses PSK modulation. Prior to modulating the voice data forwireless transmission, the voice input is run through a speechcompression circuit such as a vocoder to compress the voice input into asmaller amount of data. This reduces the amount of voice data that needsto be transmitted via the wireless network, thereby permitting the useof a smaller bit rate and a greater number of users sharing the samecommunication system.

Various vocoder techniques have been proposed and used. The most commonare various forms of linear predictive codings (LPC); for example, 2GGSM uses a RPE-LPC speech codec, while IS-95 CDMA uses a variable rateCELP codec. These predictive compression techniques are designedspecifically for voice encoding and, as such, are designed to filter outnoise and other non-speech components. As a result, the transmission ofdigital data (such as ASCII text, byte codes, binary files) can beproblematic since the vocoder processing can corrupt the digital data,making it unrecoverable at the receiving end of the transmission. Forexample, the recently introduced Qualcomm™ 4G Vocoder is a CDMA2000device that exhibits a time-varying, non-linear transfer function which,while acceptable for voice encoding, can impose significant distortionwhen attempting to transmit digital data via the vocoder.

The 4G vocoder uses the 3gpp2 standards-based EVRC-B codec having a fullrate of 9.6 kbps. Alternatively, newer vocoders may use the 3gpp2EVRC-WB or EVRC-C codec. These codecs also support lower bit rates,including a 4.8 kbps half rate and a 1.2 kbps eighth rate. These lowerrates are used when the vocoder determines that the full rate is notneeded to adequately transmit the sound signals it receives. Forexample, background noise is typically transmitted at the one-eighthrate. The EVRC-B vocoder uses these different rates to achieve a targetrate that can be controlled by the wireless carrier. For thetransmission of data via the voice channel, this can be problematicbecause the vocoder might chose less than full rate, making it difficultto successfully send non-speech data through the vocoder. For modulationtechniques such as frequency shift keying (FSK) and amplitude shiftkeying (ASK) that have been successfully used with prior generationvocoders (e.g., EVRC-A), the same frequencies and modulation bit ratecombinations that have previously worked may not reliably providetwo-way transmission of data using the newer (e.g., EVRC-B) vocoders.

SUMMARY OF THE INVENTION

The present invention provides a method of data communication using awireless communication network that allows the transmission of digitaldata over a voice channel of the communications network. In accordancewith one embodiment, the method includes the steps of:

encoding data sent in either direction between the vehicle and centralfacility using continuous signal modulation of a carrier signal suchthat the modulated carrier signal contains no more than four significantfrequency components; and

transferring the modulated carrier signal between the vehicle andcentral facility using a newer generation EVRC vocoder.

Preferably, the continuous signal modulation is carried out using eitherfrequency shift keying or amplitude shift keying, wherein the modulationbit rate and frequency(ies) are selected such that the modulated carriersignal can be decoded at the other end with a bit error rate that isless than a selected threshold. Acceptable thresholds may depend uponthe particular application, but may vary from 1% or less up to as muchas 10%.

In accordance with another aspect of the invention, there is provided amethod of exchanging data over a wireless communication system whichuses a vocoder in each direction to encode an inputted audio streamusing a vocoder that encodes speech segments by determining anapproximation of the speech segment, selecting between a full bit rateand one or more slower bit rates based at least in part on an errorcalculation relating to the difference between the approximation and thespeech segment, and generating an encoded speech segment using theapproximation and selected bit rate. The method comprises the steps of:

encoding data sent in each direction using continuous signal modulationof a carrier signal at a selected modulation bit rate and one or morefrequencies such that the vocoder selects the full bit rate based on theerror calculation;

sending the modulated carrier signal over the wireless communicationsystem;

receiving the modulated carrier signal; and

demodulating the modulated carrier signal back into the data.

If the vocoder used in this method is a newer generation EVRC vocoder,then the error calculation carried out by the vocoder will utilize aLevinson Durbin recursion, in which case the encoding step preferablyfurther comprises encoding the data using continuous signal modulationof a carrier signal at a modulation bit rate and one or more frequenciessuch that the vocoder selects the full bit rate as a result of theLevinson Durbin recursion.

In accordance with yet another aspect of the invention, there isprovided a method of exchanging data over a wireless communicationsystem which uses a vocoder in each direction to encode an inputtedaudio stream using a CELP codec that determines a predictor using aLevinson Durbin recursion that generates predictor coefficients, whereinencoding of the speech occurs at a bit rate selected at least in partbased on a prediction error that is calculated for each of a number ofiterations of the Levinson Durbin recursion. The method comprises thesteps of:

encoding first data into a first audio stream that is inputted into thevocoder used for transmission in a first direction over the wirelesscommunication system, wherein the encoding of the first data is carriedout using continuous signal modulation of a first carrier signal at afirst frequency and selected modulation bit rate such that theprediction error for the first modulated carrier signal falls below apredetermined threshold within a preselected number of iterations of theLevinson Durbin recursion;

sending the first modulated carrier signal over the wirelesscommunication system;

receiving the first modulated carrier signal;

demodulating the first modulated carrier signal back into the firstdata;

encoding second data into a second audio stream that is inputted intothe vocoder used for transmission in a second direction over thewireless communication system, wherein the encoding of the second datais carried out using continuous signal modulation of a second carriersignal at a second frequency and selected modulation bit rate such thatthe prediction error for the second modulated carrier signal falls belowthe predetermined threshold within the preselected number of iterationsof the Levinson Durbin recursion;

sending the second modulated carrier signal over the wirelesscommunication system;

receiving the second modulated carrier signal; and

demodulating the second modulated carrier signal back into the seconddata.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the invention will hereinafter bedescribed in conjunction with the appended drawings, wherein likedesignations denote like elements, and wherein:

FIG. 1 is a block diagram depicting an electronic communication systemconstructed in accordance with the invention;

FIG. 2 depicts an overview of the speech classification and ratedetermination scheme used by EVRC-B vocoders;

FIG. 3 is a flow chart of an ASK modulation technique;

FIG. 4 depicts an exemplary carrier signal modulated by continuous ASKusing a random bit pattern;

FIG. 5 is a flow chart of an FSK modulation technique; and

FIG. 6 depicts an exemplary carrier signal modulated by continuous FSKusing a random bit pattern at 10 bits/frame.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown an electronic communication system10 constructed in accordance with the invention. The communicationsystem 10 includes a conventional cellular communication network havinga voice traffic channel that is used for two-way transmission of voicedata between cellular telephones. The communication system 10 alsoincludes the ability to utilize the cellular system voice channel toexchange digital data containing information other than speech or otheraudio. As will be discussed in greater detail below, this datacommunication is carried out at least in part using continuous signalmodulation (CSM) of a carrier signal at one or more audio frequenciesselected such that when sent using a newer generation EVRC vocoder, themodulated carrier signal is transmitted by the vocoder at its full rateand can be demodulated at the receiving end such that the bit error rateis within desired, or at least acceptable, limits. This approach enablesdata communication through a newer generation EVRC vocoder in eachdirection and over a voice channel using FSK or ASK modulation withoutany significant loss of information. As used herein, “continuous signalmodulation” means modulation of a carrier signal in a manner thatproduces a resulting modulated carrier signal having no discontinuities.Also, as used herein, “newer generation EVRC vocoder” refers to an EVRCvocoder that is either an EVRC-B vocoder or newer including, forexample, an EVRC-WB or EVRC-C vocoder.

The communication system 10 includes in general a cellular communicationnetwork 12 connected to a land telephony network 14 which together areused to provide voice and data communication between a passenger vehicle20 and a call center 40. Vehicle 20 has an onboard electronics system, aportion of which is shown at 22. Electronics system 22 has a telematicsunit 23 that includes the components normally found in a cellularcommunication device, such as a CDMA compatible chipset 24 and antenna26 that enables use of the cellular network 12 to permit a vehicleoccupant to carry on voice conversations using a speaker 28 andmicrophone 30. These components of telematics unit 23 can be implementedin a conventional manner, as will be known to those skilled in the art.Apart from the microphone 30 input, onboard system 22 also includes atleast one pushbutton 32 that can be used to initiate a voicecommunication with a live advisor 42 located at the call center 40.

In accordance with 4G CDMA systems, voice data from both the vehicleoccupant (not shown) and the live advisor 42 are encoded using a vocoderto compress the speech prior to wireless transmission over the voicetraffic channel via the cell tower 16. Once received over the wirelessnetwork, the encoded speech is then decoded by the vocoder for thelistener. The vocoder is incorporated into the chipset 24 as well as ina CDMA compatible module 18 located in the base equipment at the celltower 16. Although various compression codecs can be used, in theillustrated embodiment, the 4G vocoder is implemented as a time-varying,non-linear filter. Various such codecs are well known using linearpredictive techniques; for example, a RPE-LPC codec or a fixed orvariable rate CELP codec. In the illustrated embodiment, an EVRC-B codecaccording to the 3GPP2 C.S0014-B ver. 1.0 standard (available atwww.3gpp2.org) is used, although other suitable codecs (whether linearpredictive or not) can be used in the system 10 of FIG. 1; for example,any newer generation EVRC vocoder codec, including EVRC-WB and EVRC-C.

In addition to the typical voice data transmission over the voicetraffic channel, the communication system 10 enables data communicationvia this same voice traffic channel and through the vocoder 18, 24. Thisis accomplished using a modem on either side of the vocoder; that is,using a first modem 34 incorporated into the onboard vehiclecommunication system 22 and a second modem 44 located at the call center40. These modems can have the same construction and operation so thatonly modem 34 will be described, and it will be appreciated that thedescription of modem 34 applies equally to modem 44. As shown in FIG. 1,the telematics unit 23 can switch or multiplex the CDMA 4GV chipset 24between the modem 34 and the telephony devices 28-32 so that thecellular communication network 12 can be used for either voice or datacommunication, or both, even during the same call.

Regardless of whether the cellular call is initiated at the vehicle 20or call center 40, the transmitting modem can use a predefined systemconnect tone (e.g., 850, 1778, or 2225 Hz) or series of tones to alertthe receiving modem of the requested data transmission, and the variousattributes of the data connection can then be negotiated by the twomodems. Typically, a different tone will be used in either direction. Toenable data communication over the voice channel, the modem appliescontinuous signal modulation (CSM) to a carrier signal to encode thedigital data being transmitted into a CSM carrier signal that can besuccessfully sent via the vocoder 18, 24 and over the voice trafficchannel of the cellular network 12. In the different illustratedembodiments, one or more particular forms of CSM encoding are used; forexample, frequency shift keying or amplitude shift keying. As will bediscussed farther below, encoding of the digital data is implemented bymodem 34 using one or more carrier signals that are modulated with thedata using a CSM encoder/decoder 36.

As illustrated in FIG. 1, modem 34 and its encoder/decoder 36 can beimplemented using software running on the telematics microprocessor 35.This software can be stored in the telematics memory 37. Otheralternative implementations will be apparent to those skilled in theart; for example, the modem 34 could be incorporated into the 4GVchipset 24, or can be implemented using a dedicated IC or other hardwarecomponent, or the modem software could be stored on processor 35 itselfor on other memory not shown.

On the vehicle 20, the digital data being CSM encoded and sent via modem34 can be obtained by the telematics unit 23 from one or more vehiclesystem modules (VSMs) 38 over a vehicle network 39. These modules 38 canbe any vehicle system for which information transmission is desired toor from the call center 40 or other remote device or computer system.For example, one VSM 38 can be a diagnostic system that providesdiagnostic trouble codes or other diagnostic information to the callcenter 40. As another example, VSM 38 can be a GPS-enabled navigationsystem that uploads coordinates or other such information concerning thevehicle's location to the call center. Data can be transmitted from thecall center (or other remote device or computer system) to the vehicleas well. For example, where VSM 38 is a navigation system, new maps orother directional or point of interest information can be downloaded tothe vehicle. As another example, a VSM 38 can be an infotainment systemin which new music or videos can be downloaded and stored for laterplayback. Furthermore, the term “digital data” as used herein includesnot only information, but also executable code such that new programmingcan be downloaded to the vehicle via the voice traffic channel from aserver or other computer. Those skilled in the art will know of othersuch VSMs 38 and other types of digital data for which communication toand/or from the vehicle 20 is desired.

The vehicle network 39 can be implemented as any suitable network, suchas a controller area network (CAN), a media oriented system transfer(MOST), a local interconnection network (LIN), an Ethernet, a local areanetwork (LAN), and can utilize appropriate connections and protocolssuch as those that conform with known ISO, SAE and IEEE standards andspecifications. A separate infotainment network (not shown) can also beincluded for access by the telematics unit 23 to a vehicle radio system,in which case the speaker 28 could be eliminated and instead the vehicleradio system speaker(s) used for audio output during voice conversationsthrough the communications system 12.

Land network 14 can be a conventional land-based telecommunicationsnetwork that is connected to one or more landline telephones andconnects wireless carrier network 12 to call center 40. For example,land network 14 can include a public switched telephone network (PSTN)and/or an Internet Protocol (IP) network, as is appreciated by thoseskilled in the art. Of course, one or more segments of land network 14could be implemented through the use of a standard wired network, afiber or other optical network, a cable network, power lines, otherwireless networks such as wireless local area networks (WLANs) ornetworks providing broadband wireless access (BWA), or any combinationthereof. Furthermore, call center 40 need not be connected via landnetwork 14, but could include wireless telephony equipment so that itcan communicate directly with wireless network 12.

Call center 40 includes not only the live advisor 42 and modem 44, butalso several other components. It includes a PBX switch 46 to routeincoming calls either to one or more telephones 48 for voicecommunication or to modem 44 for data transmission. The modem 44 itselfcan be connected to various devices such as a server 50 that providesinformation services and data storage, as well as a computer used by thelive advisor 42. These devices can either be connected to the modem 44via a network 52 or alternatively, can be connected to a specificcomputer on which the modem 44 is located. The various components ofFIG. 1 include some that are conventional and others that can beimplemented based upon the description contained herein and theknowledge possessed by one skilled in the art. For example, although themodems 34, 44 and their CSM encoder/decoder are not conventionalcomponents, techniques for implementing CSM encoding and decoding areknown and can be implemented by those skilled in the art using suchcomponents as DSPs and ASICs. Similarly, the other features needed toimplement the modems 34, 44 are all well known to those skilled in theart.

For the EVRC-B and other newer generation EVRC vocoders, the successfultransmission of the digital data through the vocoder can be largelydependent on the encoding and transmission rate used by the vocoder. For4G vocoders such as Qualcomm's® which use an EVRC-B codec that followsthe 3GPP2 C.S0014-B ver. 1.0 specification (available at www.3gpp2.org),different rates are used for different types of speech, tones, andbackground noise. In general, the vocoder encodes and transmits incomingdata at a rate that is determined by classifying the inputted signalinto categories representative of different types or portions of speech.These categories include voiced, unvoiced, and transient, as well assilence and up- and down-transients. Depending initially upon thisclassification, but also upon additional tests, the vocoder selects aparticular operating mode in which it uses a particular coding schemeand rate to encode and transmit the received data. Generally, thisprocess is carried out on a frame by frame basis, with each framecorresponding to 20 ms of data sampled at 8 kHz. For voicecommunications, the process is designed to provide a faithfulreproduction of speech while accommodating other communication needs(such as ring-back tones) and attempting to minimize bandwidthutilization. However, this process can significantly inhibit datacommunications over the voice channel because it can result in less thanfull rate transmission. Without full rate transmission, it can bedifficult if not impossible to transmit the digital data through theEVRC-B vocoder at a bit error rate that is acceptable for mostapplications.

For prior generation vocoders that utilize EVRC-A, an incoming signalneed only look like speech to get full rate. Thus, modulation techniquessuch as continuous FSK could be utilized to obtain full rate. For thenewer generation EVRC vocoders, however, the ability to achieve fullrate is more difficult. FIG. 2 depicts an analysis of the EVRC-B speechclassification scheme contained in the 3GPP2 C.S0014-B ver. 1.0specification, showing the different tests used to classify the incomingdata and which of those tests lead to full rate transmission. EVRC-Buses three main anchor operating points (AOP's): AOP0, AOP1, and AOP2.These operating points are used in determining the rate selection andthe anchor operating points themselves are determined based on a targetaverage rate that can be adjusted by the wireless carrier. Thus, aservice provider desiring to send digital data through the vocodertypically cannot control the anchor operating point determination.Instead, obtaining the desired full rate can be accomplished bymodulating or otherwise conditioning the encoded carrier signalaccording to one or more of the paths of FIG. 2 that lead to the fullrate determination.

In general, the process of FIG. 2 classifies the incoming data as one ofa number of categories of speech, such as transient or voiced, and basedon that categorization determines if it is to be transmitted at fullrate. As a part of the EVRC-B vocoder processing, a Levinson Durbinrecursion is applied and, regardless of the classification of the speechas transient or otherwise, an error parameter of this recursion ismonitored to determine if full rate should be assigned. In particular, aStoporder30 iteration index is calculated and if this value is less thanor equal to four, then full rate transmission is used. This allows forring back tones to be transmitted at full rate.

The Levinson Durbin recursion is used to model or approximate the frameof speech inputted into the vocoder by determining the poles of anall-pole IIR filter. This is done by multiple recursions of anautocorrelation function to determine the coefficients of the filter.After each iteration, a prediction error (normalized energy error) iscalculated that is related to the difference between the approximation(as defined by the computed coefficients) and the inputted speech. Forspeech segments that can be closely approximated using a low-orderpolynomial, the error will become quite low within a few iterations ofthe recursion. Thus, for ring back tones comprising only one or twoaudio frequencies, the prediction error will fall below a predeterminedthreshold (e.g., −30 dB) within a preselected number of iterations(e.g., 4) of the Levinson Durbin recursion. Then, by assigning full ratein this instance, the vocoder can help insure that the ring back tonesare successfully transmitted. The Stoporder30 test is used to determinewhether the inputted speech has this tonal quality to it. In particular,the Stoporder30 test determines whether the prediction error falls belowa predetermined threshold of −30 dB within four iterations of theLevinson Durbin recursion. If so, full rate CELP is used to encode theframe of inputted speech.

This feature of newer generation EVRC vocoders can be utilized to enabletransmission of digital data using a modulation technique that meets therequirements of the Stoporder30 test. One way in which to do this is toencode the digital data using continuous signal modulation (CSM) of acarrier signal such that the modulated carrier signal contains no morethan four significant frequency components. This allows the LevinsonDurbin recursion to converge at a small prediction error within the fouriterations used by the Stoporder30 test. Furthermore, with a suitableselection of the modulation bit rate and frequency components, the CSMmodulated carrier signal can be transmitted between the vehicle and callcenter or other central facility in a manner that allows the digitaldata to be decoded from the transferred modulated carrier signal.

Because the EVRC vocoder is designed to encode the phonetic componentsof speech, it does not handle all frequencies the same. Thus, whenproducing a modulated carrier signal having no more than foursignificant frequency components, suitable modulation bit rate andfrequency(ies) should be selected such that the bit error rate (BER) oftransmitted digital data is within a predetermined acceptable limit. Themaximum acceptable BER may depend on the particular applicationinvolved, since it may be less important in some data transmissionapplications than others that a certain BER maximum be met. In general,the BER is preferably no more than 10%, even more preferable is 5% orless, and most commercial applications would utilize a selection offrequency(ies) and modulation bit rate that provides a BER of 3% or lessand, most preferably, that is 1% or less.

Apart from using a modulation approach for newer generation EVRCvocoders that produces a carrier signal having no more than foursignificant frequency components, modulation of a carrier signal usingthe digital data can be carried out for any vocoder of the type thatencodes speech segments (e.g., 20 msec frames) of an inputted audiostream by determining an approximation of the speech segment, selectingbetween a full bit rate and one or more slower bit rates based at leastin part on an error calculation relating to the difference between theapproximation and the speech segment, and generating an encoded speechsegment using the approximation and selected bit rate. For suchvocoders, transmission of the data through the vocoder can beaccomplished by the steps of: encoding data sent in each direction usingcontinuous signal modulation of a carrier signal at a selectedmodulation bit rate and one or more frequencies such that the vocoderselects the full bit rate based on the error calculation;

sending the modulated carrier signal over the wireless communicationsystem;

receiving the modulated carrier signal; and

demodulating the modulated carrier signal back into the data.

The modulation bit rate and one or more frequencies can be pre-selectedbased at least in part on a bit error rate determination. This can bedone by determining one or more combinations of bit rate and carrierfrequencies such that the encoded data is sent via the vocoder and thendemodulated back into the data at a bit error rate that is below aselected threshold. The threshold can be application dependent and, asdescribed above, can be 10% or less, preferably is 5%, more preferably3% or less and, in a highly preferred embodiment, is no more than 1%.For newer generation EVRC vocoders and others that use a Levinson Durbinrecursion to determine if the error calculation indicates convergence ofthe approximation within a few iterations, this method can be carriedout by encoding the data using continuous signal modulation of a carriersignal at a modulation bit rate and one or more frequencies such thatthe vocoder selects the full bit rate as a result of the Levinson Durbinrecursion.

As a more specific example, transmission of first data in a firstdirection between the vehicle and central facility and transmission ofsecond data in the reverse direction can be carried out using a vocoderin each direction to encode an inputted audio stream using a CELP codecthat determines a predictor using a Levinson Durbin recursion thatgenerates predictor coefficients, wherein encoding of the speech occursat a bit rate selected at least in part based on a prediction error thatis calculated for each of a number of iterations of the Levinson Durbinrecursion. The newer generation EVRC vocoders operate in this fashion.The following method can be used to exchange the first and second data:

encoding first data into a first audio stream that is inputted into thevocoder used for transmission in a first direction over the wirelesscommunication system, wherein the encoding of the first data is carriedout using continuous signal modulation of a first carrier signal at afirst frequency and selected modulation bit rate such that theprediction error for the first modulated carrier signal falls below apredetermined threshold within a pre-selected number of iterations ofthe Levinson Durbin recursion;

sending the first modulated carrier signal over the wirelesscommunication system;

receiving the first modulated carrier signal;

demodulating the first modulated carrier signal back into the firstdata;

encoding second data into a second audio stream that is inputted intothe vocoder used for transmission in a second direction over thewireless communication system, wherein the encoding of the second datais carried out using continuous signal modulation of a second carriersignal at a second frequency and selected modulation bit rate such thatthe prediction error for the second modulated carrier signal falls belowthe predetermined threshold within the pre-selected number of iterationsof the Levinson Durbin recursion;

sending the second modulated carrier signal over the wirelesscommunication system;

receiving the second modulated carrier signal; and

demodulating the second modulated carrier signal back into the seconddata.

Again, the predetermined threshold can be −30 dB or other suitable valueand the pre-selected number of iterations can be 4 or more or less thanthis number for vocoders that do not follow the 3gpp2 specification.

Turning now to FIGS. 3-6, various continuous signal modulation (CSM)encoding techniques that can be used with the data transmission methodsdiscussed above will now be described. Because the vocoder used forcellular communication filters out frequencies above that needed forspeech transmission, successful data transmission over the cellularvoice traffic channel is carried out using audio frequencies at or belowseveral kilohertz. Thus, for the data modulation techniques used in thepreferred embodiments, the carrier frequency is limited to those withinthis upper frequency. Preferably, a frequency range of 300-2,200 Hz isused. Encoding of the data into the carrier signal can be done at thetransmitting end by the modem (e.g., by the CSM encoder/decoder 36 inFIG. 1), following which the modulated carrier signal is then sent tothe vocoder (e.g., to CDMA chipset 24) for transmission to the centralfacility. At the receiving end, the modulated carrier signal obtainedfrom the receiving vocoder (e.g., in the CDMA module 18) can be decodedby the receiving modem (e.g., modem 44) back into the original digitaldata. Techniques for both encoding and decoding data using the variousmodulation techniques discussed below are known to those skilled in theart.

In FIG. 3, there is shown a method for amplitude shift keying (ASK)modulation of a carrier signal using the binary data to be transmitted.FIG. 4 depicts an exemplary waveform for a sample bit pattern1001011100. Because the vocoder is designed to efficiently encodespeech, not all frequencies within the preferred range of 300-2,200 Hzwill produce the same bit error rate for a given modulation bit rate.Thus, selection of the bit rate and frequency can be done empirically bytesting various combinations of bit rate and frequency to see what biterror rate is achieved for each combination. Exemplary combinationsinclude 250 bps along with one of the following frequencies: 500 Hz, 700Hz, 1,000 Hz, and 1,500 Hz. Typically, a different frequency will beused in each transmission direction between the vehicle and centralfacility.

As discussed above, where a newer generation EVRC vocoder or the like isused that will provide full rate for near-pure tonal data such as ringback tones, the ASK modulation should not involve zero or near-zeroamplitudes in the modulation since this will produce discontinuities inthe carrier signal. Thus, continuous signal modulation (CSM) using ASKinvolves modulating the amplitude between two non-zero values and in amanner that provides a continuous carrier signal, as shown in FIG. 4. Inthis way, discontinuities in the modulated carrier signal can be avoidedso that the newer generation EVRC vocoder Stoporder30 test will show thedesired convergence within 4 iterations, thereby assuring full rate.

FIG. 5 depicts a method of frequency shift keying (FSK) modulation. Asis known, in FSK, the carrier signal is modulated between twofrequencies—in this case frequency no. 1 corresponds to a binary 1 andfrequency 2 to a binary 0. An exemplary plot is shown in FIG. 6 for asample bit pattern 1101001010 at a bit rate of 500 bps, with frequency 1being 300 Hz and frequency 2 being 750 Hz. For transmission in thereverse direction a different pair of frequencies are preferably used.For a sampling rate of 160 samples at an 8 kHz sampling frequency, theseten sample bits represent a typical 20 ms frame of data, such as is usedin CDMA. As with ASK, the FSK modulation is done in a manner thatproduces a continuous signal with no discontinuities that would preventthe Stoporder30 test from assigning full rate when sent through a newergeneration EVRC vocoder.

The determination of desired or acceptable frequency pairs for eachdirection of data transmission can be done by testing using actualvocoders to encode and then decode the modulated carrier signal, with achecksum or other error detection and/or correction being used todetermine the bit error rate. A frequency sweep in increments of, forexample, 50 Hz can be used for each particular modulation bit ratetesting. Thus, for example, for a bit rate of 500 bps, the firstfrequency can be set at, e.g., 300 Hz and a range of frequencies for thesecond frequency tested in the range of 400-2,200 Hz, each timeincrementing by 50 Hz and determining the bit error rate. Thereafter,the first frequency can be incremented to 350 Hz and the processrepeated. This empirical testing results in a set of frequency pairs andresulting BER can be determined for any particular modulation bit rate.From this, a desired or acceptable combination of modulation bit rateand frequency pairs can be selected for each direction of datatransmission. Preferably, the encoding (modulation) bit rate used is inthe range of 200-800 bps for continuous FSK. Also, the frequency pairsselected preferably maintain a minimum frequency separation from eachother of at least 150 Hz and more preferably 250 Hz. In this regard,where the vehicle to central facility communication protocol usesterminating system connect tones, the selected frequency pairspreferably maintain at least 100 Hz and more preferably at least 200 Hzof frequency separation between the connect tone and frequency pair inany one direction. As one specific example, the modulated carrier signalsent from the vehicle to the call center uses a bit rate of 500 bps anda frequency pair of 650 Hz and 1,150 Hz with a terminating systemconnect tone between the modems of 850 Hz, whereas in the otherdirection (call center to vehicle) the data is encoded also at 500 bps,but using a frequency pair of 900 Hz and 1,500 Hz with a system connecttone of 2,225 Hz.

It is to be understood that the foregoing description is of one or morepreferred exemplary embodiments of the invention. The invention is notlimited to the particular embodiment(s) disclosed herein, but rather isdefined solely by the claims below. Furthermore, the statementscontained in the foregoing description relate to particular embodimentsand are not to be construed as limitations on the scope of the inventionor on the definition of terms used in the claims, except where a term orphrase is expressly defined above. Various other embodiments and variouschanges and modifications to the disclosed embodiment(s) will becomeapparent to those skilled in the art. All such other embodiments,changes, and modifications are intended to come within the scope of theappended claims.

As used in this specification and claims, the terms “for example” and“such as,” and the verbs “comprising,” “having,” “including,” and theirother verb forms, when used in conjunction with a listing of one or morecomponents or other items, are each to be construed as open-ended,meaning that that the listing is not to be considered as excludingother, additional components or items. Other terms are to be construedusing their broadest reasonable meaning unless they are used in acontext that requires a different interpretation.

1. A method of sending data between a vehicle and central facility overa wireless communication system, comprising the steps of: encoding datasent in either direction between the vehicle and central facility usingcontinuous signal modulation of a carrier signal such that the modulatedcarrier signal contains no more than four significant frequencycomponents; and transferring the modulated carrier signal between thevehicle and central facility using a newer generation EVRC vocoder. 2.The method set forth in claim 1, further comprising the step of decodingthe data from the transferred modulated carrier signal.
 3. The methodset forth in claim 2, wherein the encoding step further comprisesencoding the data at a selected bit rate and one or more selectedfrequencies such that the decoded data has a bit error rate of no morethan 10%.
 4. The method set forth in claim 2, wherein the encoding stepfurther comprises encoding the data at a selected bit rate and one ormore selected frequencies such that the decoded data has a bit errorrate of no more than 3%.
 5. The method set forth in claim 2, wherein theencoding step further comprises encoding the data at a selected bit rateand one or more selected frequencies such that the decoded data has abit error rate of no more than 1%.
 6. The method set forth in claim 2,wherein the encoding step further comprises: generating the modulatedcarrier signal sent from the vehicle to the central facility usingfrequency shift keying modulation at a first bit rate that produces afirst continuous carrier signal modulated in accordance with the databetween first and second frequencies that are selected in conjunctionwith the bit rate such that the decoded data has a bit error rate of nomore than 5%; and generating the modulated carrier signal sent from thecentral facility to the vehicle using frequency shift keying modulationat a second bit rate that produces a second continuous carrier signalmodulated in accordance with the data between third and fourthfrequencies that are selected in conjunction with the bit rate such thatthe decoded data has a bit error rate of no more than 5%.
 7. The methodset forth in claim 6, wherein the first and second bit rates are no morethan 800 bps, and wherein the first, second, third, and fourthfrequencies are different than one another and are each within the rangeof 300 Hz to 2,200 Hz.
 8. The method set forth in claim 7, wherein thebit rates and frequencies are selected such that the bit error rate ofdecoded data sent between the vehicle and central facility in eitherdirection is no more than 1%.
 9. The method set forth in claim 7,wherein the first frequency is 650 Hz±50 Hz, the second frequency is1150 Hz±50 Hz, the third frequency is 900 Hz±50 Hz, and the fourthfrequency is 1500 Hz±50 Hz.
 10. The method set forth in claim 2, whereinthe encoding step further comprises: generating the modulated carriersignal sent from the vehicle to the central facility using amplitudeshift keying modulation that produces a first continuous carrier signalat a first frequency modulated in accordance with the data between firstand second non-zero amplitudes; and generating the modulated carriersignal sent from the central facility to the vehicle using amplitudeshift keying modulation that produces a second continuous carrier signalat a second frequency modulated in accordance with the data betweenfirst and second non-zero amplitudes.
 11. The method set forth in claim10, wherein each of the generating steps further comprise encoding thedata in the modulated carrier signal at a bit rate that is selected inconjunction with the frequency such that the decoded data has a biterror rate of no more than 5%.
 12. The method set forth in claim 10,wherein the first and second frequencies are different than one anotherand are each within the range of 300 Hz to 2,200 Hz.
 13. The method setforth in claim 1, further comprising the step of determining if themodulated carrier signal contains no more than four significantfrequency components by performing a Levinson Durbin recursion in thevocoder, calculating a prediction error after each iteration of therecursion, and determining if the prediction error falls below apredetermined threshold within four iterations of the recursion.
 14. Amethod of exchanging data over a wireless communication system whichuses a vocoder in each direction to encode an inputted audio streamusing a vocoder that encodes speech segments by determining anapproximation of the speech segment, selecting between a full bit rateand one or more slower bit rates based at least in part on an errorcalculation relating to the difference between the approximation and thespeech segment, and generating an encoded speech segment using theapproximation and selected bit rate, wherein the method comprises thesteps of: encoding data sent in each direction using continuous signalmodulation of a carrier signal at a selected modulation bit rate and oneor more frequencies such that the vocoder selects the full bit ratebased on the error calculation; sending the modulated carrier signalover the wireless communication system; receiving the modulated carriersignal; and demodulating the modulated carrier signal back into thedata.
 15. The method set forth in claim 14, further comprising the stepof pre-selecting the modulation bit rate and one or more frequenciesbased at least in part on a bit error rate determination.
 16. The methodset forth in claim 15, wherein the step of pre-selecting the modulationbit rate and one or more frequencies further comprises pre-selecting themodulation bit rate and one or more frequencies such that data that isencoded using the pre-selected modulation bit rate and one or morefrequencies, sent via the vocoder, and then demodulated back into thedata has a bit error rate that is below a selected threshold.
 17. Themethod set forth in claim 16, wherein the selected threshold is 5%. 18.The method set forth in claim 16, wherein the selected threshold is 3%.19. The method set forth in claim 16, wherein the selected threshold is1%.
 20. The method set forth in claim 14, wherein the error calculationcarried out by the vocoder utilizes a Levinson Durbin recursion, andwherein the encoding step further comprises encoding the data usingcontinuous signal modulation of a carrier signal at a modulation bitrate and one or more frequencies such that the vocoder selects the fullbit rate as a result of the Levinson Durbin recursion.
 21. The methodset forth in claim 14, wherein the encoding step further comprises:generating the modulated carrier signal sent from the vehicle to thecentral facility using frequency shift keying modulation at a first bitrate that produces a first continuous carrier signal modulated inaccordance with the data between first and second frequencies; andgenerating the modulated carrier signal sent from the central facilityto the vehicle using frequency shift keying modulation at a second bitrate that produces a second continuous carrier signal modulated inaccordance with the data between third and fourth frequencies; whereineach of the four frequencies are separated from each other by at least150 Hz.
 22. The method set forth in claim 21, wherein the fourfrequencies are separated from each other by at least 250 Hz.
 23. Themethod set forth in claim 21, further comprising the step sending afirst system connect tone from the vehicle to the central facility andsending a second system connect tone from the central facility to thevehicle, wherein the first system connect tone utilizes a fifthfrequency that is separated from the first and second frequencies by atleast 200 Hz, and wherein the second system connect tone utilizes asixth frequency that is separated from the third and fourth frequenciesby at least 200 Hz.
 24. The method set forth in claim 14, wherein theencoding step further comprises: generating the modulated carrier signalsent from the vehicle to the central facility using amplitude shiftkeying modulation that produces a first continuous carrier signal at afirst frequency modulated in accordance with the data between first andsecond non-zero amplitudes; and generating the modulated carrier signalsent from the central facility to the vehicle using amplitude shiftkeying modulation that produces a second continuous carrier signal at asecond frequency modulated in accordance with the data between first andsecond non-zero amplitudes.
 25. A method of exchanging data over awireless communication system which uses a vocoder in each direction toencode an inputted audio stream using a CELP codec that determines apredictor using a Levinson Durbin recursion that generates predictorcoefficients, wherein encoding of the speech occurs at a bit rateselected at least in part based on a prediction error that is calculatedfor each of a number of iterations of the Levinson Durbin recursion, themethod comprising the steps of: encoding first data into a first audiostream that is inputted into the vocoder used for transmission in afirst direction over the wireless communication system, wherein theencoding of the first data is carried out using continuous signalmodulation of a first carrier signal at a first frequency and selectedmodulation bit rate such that the prediction error for the firstmodulated carrier signal falls below a predetermined threshold within apreselected number of iterations of the Levinson Durbin recursion;sending the first modulated carrier signal over the wirelesscommunication system; receiving the first modulated carrier signal;demodulating the first modulated carrier signal back into the firstdata; encoding second data into a second audio stream that is inputtedinto the vocoder used for transmission in a second direction over thewireless communication system, wherein the encoding of the second datais carried out using continuous signal modulation of a second carriersignal at a second frequency and selected modulation bit rate such thatthe prediction error for the second modulated carrier signal falls belowthe predetermined threshold within the preselected number of iterationsof the Levinson Durbin recursion; sending the second modulated carriersignal over the wireless communication system; receiving the secondmodulated carrier signal; and demodulating the second modulated carriersignal back into the second data.